1. Field of the Invention
The present invention relates to a flat panel display, and in particular to a surface light source device thereof providing discontinuous light intensity distribution.
2. Description of the Related Art
Flat panel displays (FPD) are widely applied in the fields of information, communication, transportation, and consumptive electronic products due to their high resolution, light weight and compact size, low voltage and low power consumption. The related products are large-screen-sized high definition television sets (HDTV), wall-hung type television sets and projecting television sets, and projection television sets, notebook computers, PDA (personal digital assistant), GPS (global positioning system), electronic dictionaries, and the like.
When a liquid crystal display is used in low or dim light, internal illumination is usually provided by a surface light source of side light type fitted with a light guide plate. For example, a conventional surface light source device of side light type 1′ covers a reflective liquid crystal panel 2′, as disclosed in FIG. 1. FIG. 2 discloses a conventional surface light device of side light type 1′ stacked in a transmissive liquid crystal panel 3′. The surface light source mainly comprises a light guide plate 11′ and a light source 12′. The light source 12′ is placed along an end surface 111′ of the light guide plate 11′. As shown in FIG. 1, the reflective liquid crystal panel 2′ comprises, from top to bottom, an upper polarized plate 21′, an upper substrate 22′, a liquid crystal cell 23′, a reflective layer 24′, and a lower substrate 25′, and further the surface light source 1′ covering the upper polarized plate 21′. As shown in FIG. 2, the transmissive liquid crystal panel 3′ comprises, from top to bottom, an upper polarized plate 31′, an upper substrate 32′, a liquid crystal cell 33′, a lower substrate 34′, a lower polarized plate 35′, a diffusion plate 36′, the surface light source 1′ device, and a reflective plate 37′.
Light emitted by the light source 12′ enters the light guide plate 11′ from the end surface 111′ and undergoes total reflection inside the light guide plate 11′, according to Snell's Law. The micro-prism 112′ structure on the top of the light guide plate 11′ reflects and directs the light to the emitting face 113′ of the bottom of the light guide plate 11′ for emission, such that light emitted therefrom exhibits a uniform brightness distribution in the total area. Then, as shown in FIGS. 1 and 2, the light is further collected uniformly into the desired display ranges of the liquid crystal panel 2′ or 3′ (reflective and transmissive types, respectively). In addition, light adjusters, such as micro-reflectors or diffusers, may be set in the light guide plate to improve uniformity of light intensity from the light guide plate. FIGS. 3 and 4 show a graph of distribution density of the light adjusters versus the distance between the light adjusters and the light source and a graph of the property of the light output from the light guide plate.
The improvement of light guide plate and surface light source device performance is mostly effected by the design of the liquid crystal panel working together. Especially, for liquid crystal panel development, early reflective and transmissive types have evolved into the transflective type, and, in turn, applications currently gradually developed in which pixels with different light transmittivities are set in different areas in the display range of the liquid crystal panel. For example, in a panel structure of a single liquid crystal layer with duplex image function, the liquid crystal panel has a transmissive area and a reflective area, in which a side image of the liquid crystal panel is formed by light transmitted from a surface light source through the transmissive area, and another side image formed by light from a surface light source reflected by the reflective area. However, surface light source devices equipped with conventional light guide plates purposely provide a surface light illumination of uniform brightness in all areas. When the surface light illumination of uniform brightness works together with liquid crystal panels having areas of different light transmitivity as mentioned above, brightness is not uniform in the display range of the liquid crystal panel.